Light sensitive electrode and method of manufacture



Aug.

R. B. JANEs LIGHT SENSITIVE ELECTRODE AND METHOD OF MANF'ACTURE Filed April'o, 1940v JNVENTOR. POBERT B. JANES ATTORNEY.

PatentedAug. 19, 1941 UNITED. STATES lafrlezlvr N OFFICE LIGHT SENSITIVE ELECTRODE AND METHOD QF MANUFACTURE Robert B. Janes, Verona, N. J., assgnor to Radio Corporation of America, a corporation of Dela- Ware v Application April so, 1940, ser'iiu No. 332,427

15 Claims.

My invention relates to electrode assemblies including semi-transparent metal lms and in particular to the.manufacture and application of such assemblies and Iilms to television trans-- mitting devices. 4

In conventional television transmitting tubes of the type described by Iams and Rose, Serial No. 276,106, led May 2'7, 1939, light is'focusedthrough a thin semi-transparent electrically conducting electrode which serves as the signal electrode for the tube, and upon a photosensitive mosaic of mutually insulated particles. I have found that the transmission of light through such a semi-transparent conductor is considerably less'than that which might normally be expected by reason of the absorption of the semi-transparent lm.

It is an object of my invention to provide a semi-transparent metal film and method of manufacturing such a film' and associated structure which has a higher light transmission factor than heretofore obtainable.' It is a further object to provide an improved light sensitive electrode structure of the semi-transparent type in which the light incident upon the structure `is more These and other objects, features andy advantages` of my invention will become. apparent when considered in view of the following description and the accompanying drawing in which:.-

Figure 1 is a longitudinal sectional view of a television transmitting tube incorporating an `electrode made in accordance with my invention and Figure 2 is a greatly enlarged cross-sectional view of a portion of the metal film and associated electrode structure as used in the tube of Figure 1.

Referring to Figure 1 which shows a tube of the type described by Iams and Rose in the above-mentioned co-pending application and incorporating my novel electrode structure. the

tube comprises an evacuated envelope l enclosing at one end a target or mosaic electrode 2 and at the opposite end an electron gun assembly 3 adapted to'project electrons toward the mosaic electrode. While I will describe my improved electrode assembly in connection with a photoelectrically sensitive mosaic structure, it is to be understood that my invention is equally appli- -eiliciently utilized.- It is a still further object cable to semitransparent electrodes utilizing light 4sensitive lstructure other thanthe mosaic type.

My invention is thus equally applicable to electrodes utilizing semi-conductive and voltaic-type` light sensitive-structures.

The velectron gun assembly 3 is of the convenl tional type comprising an electron source such as an indirectly heated cathode, a control electrode and one or more electron beam forming anodes. The electrons from the cathode are directed as an electron beam toward the mosaic electrode 2 and controlled in their passage to themosaic electrode by a pair of deflection plates 4, a'pairof magnetic deflection-coils 5 operating to deiiect the beam in a uniform magnetic field such as produced by the elongated magnetic coil 6.

The mosaic electrode 2 which faces the electron gun preferablyv` includes a transparent sheet of insulation such as the mica sheet 'l having on its surfacefacing the electron gun 3 a great number of mutually separated and insulatedphotosensitive ,particles 8. In operation, anoptical.

image such as represented by the arrow y9 is focused by aconventional lens system I0 through the mica sheet 1 and upon the photosensitive particles 8 which are exposedto and scanned by theelectron beamfrom the electron gun 3.4-

Inaaccordance with my invention, and as best shown in Figure 2, the mica sheet 1 is provided with a metal film Il which is thin enough to be semi-transparent to light i by vaporizing a quantity of metal in a highly rareiied atmosphere with respect to oxidizing gases and corresponding to a pressure of less than lllfimm.` Hg and condensing the metal on the mica sheet 1. While I refer to these vaporizing and condensing "steps as being performed in a rareed oxidizing atmosphere, the film I I may be deposited such as by sputtering, which is a form of evaporation and condensation, in a non-oxidizing atmosphere such as highly puried hydrogen. Therefore, when I refer to evaporation of metal in a high vacuum, I am referring to a highly rareiied atmosphere with, respect to oxidizing gases; Following' the formation of the metal lm Il, I vaporize and condense aquantity of transparent material toform a transparent layer I2 on the exposed lsurface of the filmy II. This latter evaporation andvcondensation is preferably performed'in the absence of any gas whatsoever at a vacuum corresponding to a pressure of -6 mm. Hg, but this pressure may be increased diethickness Vbetween 75and 125 units with av normal light transmission of 50% may be formed in such a high vacuum. I have found, however, that under poor vacuum conditions with respect to an oxidizing atmosphere the film of metal has low electrical conductivity and-is more opaque to light thanwhen deposited in a high vacuum.

Following the formation of the film II, I evaporate material to form the layer 4I2 without 'removing the foundation or mica'sheet 1 from the vacuum chamber or bell jar.v Provided the. film II was deposited in a high vacuum with respect to all gases, the layer I2 material may be evap-Y orated immediately, while maintaining the high vacuum. However, if a sputtering process wasused, all non-oxidizing gas is removed from the chamber or bell 4jar by evacuating to a pressure of 10 6 mm. Hg or less. I have found,that.if the metal lm II and layer I2 are not deposited in this manner, the results obtained are not as satis# factory in -that the metal iilm tends to change color and becomes more opaque and that the layer I2 is not firmly adherent to the metal film II. However, if the film II and layer I2 are successively vdeposited without admitting air or an oxidizing gas to the vacuum enclosure, the

layer I2 is in molecular contact with the 'film II and the best results are obtained.

. I choose a material for the layer I2 havingI an index of refraction between 1.7 and 2.2, the exact index being determined by the particular metal of which the lm II is composed.' For platinum or' palladium this index of refraction may be.

from 1.75 to 2:0, whereas the index of refraction may be from 2.0 to 2.2 for a film II of a metal such as silver.' These indices of refraction have beenfound suitable for-metal film thicknesses having a normal white'light transmission of 50%. By normal white light transmission I mean the proportion of incident light which is trans-v mitt'ed through the lm prior to the deposition of the transparent layer I2. A normal white light transmission factor of approximately 50% corresponds to V a film thickness of between 75 and 12,5 units. The index of refraction of the material of the layer, I2 is on the high side .of the above limits for lthicker metal films and `on the low side of' theabove limits for thinner metal films.' I have found materials such as aluminum oxide, magnesium oxide and bismuth oxide to be particularly advantageous forisuch' yfilms because their indices of refraction liev within the above limits for use with platinum .or"pal1adium films when the layer I2 is deposited by vaporization in layer I2 over a platinum or palladium film, provided the material of the layer I2 is vaporized and deposited in a vacuum somewhat poorer than the best vacuum obtainable. Thus materialssuch as titanium oxide or zirconium oxide and having a higher index of refraction than 2.0 may be deposited as a layer having an index of refraction within the range of 1.75 to 2.o when deposited by condensation in vacuum corresponding to a pressure of approximately 1 micron Hg. Thus between the steps of depositing the film II and layer I2, a slight amount of air or other gas may be admitted to the vacuum chamber or bell jar to a pressure of approximately 1 micron Hg, a1- though I prefer to introduce a non-oxidizing gas in this step. This pressure may be further increased to decrease the index of .refraction of the material when materials for the layer I2 having higher indices of refraction are used. Determinations of the indices of refraction of materials suitable for the layer I2 from bulk materials and from condensed layers indicate that the index of refraction of a condensed layer is substantiallyv refraction up to 1.85 may be made by using a l conventional refractometer, although the International Critical Tables, McGraw-Hill, New York city, gives Yreliable data on the index of refraction of, such materials.

' By the use of the layers of vtransparent ma- I terial referred to above I have obtained an increase in :the normal Whit light transmission of greater than 20% when the layer is deposited to an optical thickness of approximately 1500 angstroms. 'Ihe optical thickness of the layer I2 may be defined as the product of the actual thickness and the index of refraction ofthe layer I2, and I will use the term optical thickness to ienne the layer I2 in terms of its actual thickness and index 'of refraction. Thus if the desired transmission is tol be a maximum for white vlight whose average wave length is 6000 angstroms, the'optical thickness should be 4 or 1500 angstroms, and the actual thickness of the layer I2 for `an index of refraction of 2.0 should be 2 l or 750 angstroms. .This relation between actual thickness and optical thickness is discussed atconsiderable length by Jenkins and White in their ybook entitled Fundamentals of Physical Optics, 1937, McGraw I-Iill, New York city. With anormal transparency of 50%. the use 'of the layer l2 increases the transmission to approxie mately 60% of the incident light. Thus the light transmitting property of the film II increases .beyond the normal transmission with increased thickness of the layer I2, but if 'the layer I2 is made too thick, the light transmitting property v-of the iilm II decreases to a value `below the a high vacuum., I have found, however, that .normal transmission. Therefore, in depositing the layer I2 I continuously observe the transmission of light through the nlm by using al `standard light source with a calibrated photocell on opposite sides of the mica sheet and discontinue the vaporization when the light transmise sion hasv reached amaximum. While I have metalssuch as' silver' may be used to form the I5 IfOund an'optical thickness of 1500 angstroms to be particularly suitablaand this thickness is equal to one quarter of the average wave length ,of.white light, the optical thickness of the layer may` be equal to any odd quarter wave length 'of the light for which maximum transmission is desired. The optical thickness of the film should be uniform within a fraction of a quarter waveI length and such uniformity may be obtained by evaporating the material from a plurality of sources or from a` single source at a distance such that the angle at the source made by lines drawn to opposite sides Vof the area to be coated l with the metal filmis 20 degrees or less. If a layer thickness greater than one-quarter wave length is inadvertently deposited upon the inetal nlm Il, the vaporization and condensation may be continued until the desired thickness is obtained, although I prefer to limit the thickness 'to one-quarter wave length because additional index of refraction of 2.0.

While I have indicated the preferred embodilments of my invention of which I- am now aware and its application to light. sensitive electrodes of the mosaic type and have --also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed, such as its application to light sensitive electrodes of the continuous photosensitive type, without departing from the scope of myv invention as set forth in the appended claims.'

1. An electrode for an'electron discharge device comprisng a semi-transparent metal lrr'i, a light sensitive coating adjacent and co-extensive with one side of said metal film and a transparent coating of material on 'the opposite side of said metal film having an optical thickness equal to an odd multiple of 1500 angstroms to increase the. normal light transmitting properties of said metal lm.

2. An electrode comprising a light `sensitive member, a thin semi-transparent metal film having one side thereof adjacent said light sensitive member and a transparent closely adherent layer of transparent material in intimate contact withy the other side of said metal film,

vsaid layer having an optical thickness equal to an odd multiple of 1500 angstroms and having a thickness uniform over the area of said layer y to within asmall fraction of 1500angstroms.

s member, a thin metal nlm having a White light transmission of at least 50% having one side thereof adjacent said light sensitive member, and a transparent closely adherent layer of transparent material having an index of refraction between the limits 1.7 to 2.2 on the other side of said metal fihn, said layer being of such thickness as to produce a purple color -by reflected light when subjected 'to white light radiation.

5. An electrode including a light sensitive member, a 'metal film having a thickness between 75 and 125 angstroms co-extensive with said light sensitive member, and a layer of material having an index of refraction proportional to the thickness of said metal lm between the limits of 1.7 for a illrn 75 angstroms thick, to 2.2 for a film 125 angstroms thick, said layer having the property of increasing the light transmission of said lm for light projected through said layer.

6. An electrode including alight sensitive pho-- to-emissive member, a metal lm cna-extensive with said member having a thickness insuflicient to absorb more than 50% of light incident thereon, a transparent layer of material having a'n index of `refraction between 1.7 and 2.2 when deposited on said film, said layer being in molecular contact with said lm and having an optical thickness equal to 'odd quarter wave lengths cf light for which said film is of insuibcient thickness to absorb morethan 50% of the light incident thereon.

3. An ,electrode havingv a wh-ite `iight transmission of approximately of white light incident thereon comprising a metal film having' thickness .of substantially 1500 angstroms coextensive with said metal film and in intimate contactwith the entire surface of; one side of said nlm whereby said ynormal transmission is increased to 60%.

' 4. An electrode comprising a light sensitive 7. An electrode as claimed in claim wherein the optical thickness of said layer is equalto odd `multiples of 1500 angstroms.

8. In combination, a thin metal lm of insufflcient thickness to absorb more than 50% of incident whilte light and a layer of transparent material of such thickness as to have a purple color when viewed by reflected White light on and in intimate contact with said film, said layer having anl index of refraction'between 1.7 and 2.2.

9. In combination, a'thin metal film having a normal white light transmission of approximately l 50% and a layer of transparent material on and in molecular contact with said `lm, the material of said layer havingoan index of refraction lying between 1.7' and 2.2 and the optical thickness of said layer being approximately 1500 angstroms.

10. An elect'rode including a semi-transparent film of metal selected fromv the group consisting' of platinum and palladium, and a transparent layer of metal oxide of a metal whose oxide has an index of refraction lying between 1.75 and 2.0

on and in intimate molecular contact with said4 film of metal.

11. The method of manufacturing a semi.

transparent member including a metal film comprising the steps of vaporizing a metal in a high vacuum. condensing the vaporized metal on a substantially transparent foundation to form a semi-transparent metal film, vaporizing a mate-` rial which when: deposited as a film of 1500A angstroms thickness is transparent, said matey.rial having an index of refraction lying between 1.7 and 2.2, and dep'ositing'said vaporlzed mate-. rial on said metal film to a predetermined trans parent` thickness While maintaining lsaid vacuum substantially constant.

l2. 'Ihe method of increasingthe light transmitting properties of a metal. nlm which coinprises depositing on a substantially transparent `foundation. by vaporization and condensation a.

send-'transparent metal lm in a high vacuum, maintaining said vhigh vacuum, depositing in 4 t molecular contact with said lm a metal oxide which when deposited as a lm of 1500 angstroms thickness is transparent, said material having an index of refraction lying between 1.7 and 2.2 and discontinuing said deposition when the light A transmitting property of said metal lm has reached a miximum.

13. The method of manufacturing a semi- `transparent-assembly including a thin metal film comprising the steps of evacuating a chamber 'to provide. a rareed oxidizing gas pressure of less than 10-6 mm., Hg, vaporizing and condensing a lm of metal Within said chamber to form a thin semi-transparent metal lm on a substantially transparentl foundation, retaining said metal film in said chamber anddetermining the light transmitting property of said film, providing an absolute gas pressure slightly above 10-6 mm Hg, depositing by vaporization and condensation a layer of transparent material on said lm and discontinuing the deposition of said material when the light transmitting property of said lm has reached a maximum. l 14. The method of manufacturing a semitransparent member including a thin metal film comprising the steps oi' .vaporizing and condensing, a-quantity of metal on a supporting foundation in a rareed atmosphere corresponding to a pressure of approximately lll-js Hg to form a thin metal film, determining the light transmitting property of said metal film, slightly increasing the A,pressure of said rarefied atmosphere, depositing by vaporization and condensation a layer'of transparent material on said film and discontinuing the deposition of said material when the light transmitting property of said film has reached a maximum.

Y 15. The method of manufacturing a semitransparent electrode including a thin metal iilm comprising the steps of vaporizing and condensing a quantity of metal in a high vacuum corresponding to a highly rarefied oxidizing atmosphere to form a thin metal lm on a supporting s foundation, determining the light transmitting Y properties of said film, evaporating and condensing a layer of material having a predetermined indeiner refraction on said nlm, and controlling the degree of vacuum during the condensation of said material to produce a layer on. said film having an index of refraction lower than the index of refraction of saidmaterial prior to vaporizing. ROBERT B. J ANE'S. 

